This invention relates to a process for preparing copolymers of alkenyl aromatic monomer and alkyl half esters of maleic anhydride and more particularly to a process for preparing high molecular weight species of such copolymers.
Alkenyl aromatic monomer (e.g. styrene and substituted styrene)--maleic half ester copolymers and polymerization processes for their preparation are known. Such polymers, including alkaline salt solutions thereof, are used in coatings (e.g. textile), adhesive applications and as photoresist binders. A further use is as paper sizing agents to improve properties of the paper, such as strength, ink retention, printability and the like. In such paper coating applications, the alkyl half ester component of the polymer is important to the capability of the polymer to form a film on the paper surface. High polymer molecular weight (&gt;110,000 Daltons weight average) and lack of color in solutions of the polymer are additional important properties deemed indispensable for such paper sizing applications. Adequate molecular weight has been achieved in the past by bulk polymerizing in two stages--initially to a very viscous syrup followed by further polymerization to a solid in a plate and frame press. Before deposition on the paper, the solid high molecular weight polymer is dissolved in aqueous ammonia or caustic soda. Such a multi-step process using an antiquated plate and frame press is undesirably labor intensive.
Also disclosed in the prior art to form these types of polymers are so-called "solvent non-solvent" processes. Note, for example, U.S. Pat. No. 3,085,994--col. 1, lines 20-27. In such a process the monomers forming the polymer are soluble in the solvent polymerization medium but the polymer is insoluble so that once formed during polymerization it immediately precipitates out of solution. These polymerization systems, however, are incapable of routinely providing the high molecular weight (&gt;110,000 Daltons) required for the noted paper sizing applications.
It would be desirable to provide a process for preparing high molecular weight styrene and substituted styrene maleic half ester copolymers which is more tractable than those in the prior art.